This invention relates to the measurement of thickness and thickness distribution of a transparent film and film thickness control.
More particularly, this invention relates to a method/apparatus for measuring the film thickness of an outermost surface, to a flattening apparatus and to a process control method for wafers in film-forming steps or wafers in surface flattening processes after film-forming steps. For example in methods or production lines for manufacturing semiconductor devices on silicon wafers.
Other examples of transparent films in addition to those mentioned above are resist films or insulating films in steps for manufacturing thin film devices such as DVD, TFT and LSI reticules.
Semiconductor devices may for example be manufactured by forming a device and an interconnection pattern on a silicon wafer through the processes of film-forming, light exposure and etching. In recent years, in order to achieve higher precision and higher densities, interconnection patterns formed on silicon wafers are tending towards greater fineness and multiple layers. The forming of multiple layers of fine patterns is leading to increasing numbers of imperfections on wafer surfaces. If there are large numbers of imperfections on wafer surfaces, when fine interconnection patterns on wafers are exposed to light, it is difficult to expose the fine patterns with good dimensional and contour precision. One method used to resolve this problem consists of flattening a wafer surface on which a protecting film or insulating film is formed on a multilayer interconnection layer.
The flattening process uses CMP (Chemical Mechanical Polishing), which flattens the surface by polishing it by a chemical and physical action. CMP is a well-known process in this technical field.
An important topic in the CMP process is that of film thickness control. In the prior art, this was controlled by the process time. After the CMP process, when measurements were actually made with an ordinary film thickness measuring apparatus, a pattern (dummy pattern) of sufficient size to be easily measured by the film thickness measuring apparatus, and which was for example formed on the periphery of the chip, was measured. Further, the measurement of film thickness was performed after completing the process, washing and drying.
Japanese Unexamined Patent Publication Hei 6-252113 and Japanese Unexamined Patent Publication Hei 9-7985 disclose an in-situ measuring system capable of measuring film thickness on an actual device pattern (a fine circuit pattern on an actual product). In Japanese Unexamined Patent Publication Hei 6-252113, in the measurement of film thickness on an actual device pattern, the spectral distribution of interference light from white light due to the film is frequency-analyzed, the relation between frequency components having this spectral distribution waveform and film thickness is examined, and an absolute value of film thickness is thereby computed. On the other hand, in Japanese Unexamined Patent Publication Hei 9-7985, the change with processing time of the interference light intensity from a laser (single wavelength) is detected, and the film thickness is computed from frequency components having this waveform.
In general, in film thickness control by process time of CMP, as the polishing amount (polishing rate) per unit time varies, and due to the fact that the polishing rate is different according to the proportion of a pattern formed on a wafer in one plane (referred to hereafter as pattern surface area factor), it was difficult to perform precise film thickness control. When measurements were performed, the outermost surface film thickness was almost always different as the pattern surface area factor on the dummy pattern was different from that on an actual device pattern. If film thickness was measured after washing and drying, some time was required and this led to a decrease of throughput (FIG. 2). In laminated patterns, interconnection pattern thickness and inter-pattern volume, for example, could not be precisely controlled on the dummy pattern. Further, when examining for defects in an actual device pattern, it was difficult to perform an examination in the film thickness direction.
In the method described in Japanese Unexamined Patent Publication Hei 6-252113, although it depends on the detection wavelength region of the white light, the measurement precision on an actual device pattern is xc2x150 nm and the film thickness cannot be computed with high precision. On the other hand, in the method described in Japanese Unexamined Patent Publication Hei 9-7985, the absolute value of film thickness cannot be found with one measurement.
It is therefore an object of this invention to provide a method and apparatus which can measure film thickness and film thickness distribution of a transparent film to a precision of at least xc2x120 nm, and preferably at least xc2x110 nm, in an actual device pattern for example, and to provide a method and apparatus for manufacturing a thin film device using this technique.
As an example of this, it is an object of this invention to provide a measurement method and apparatus which, instead of performing measurements on a dummy wafer which has a different polishing rate from an actual device pattern, or on a dummy pattern formed on a product wafer of sufficient size to be measured by a prior art film thickness measuring apparatus, are able to measure absolute values of film thickness on the outermost surface layer of an actual product device pattern to a high precision, to provide a method and apparatus which allow high precision film thickness control by measuring the film thickness on the outermost surface layer of an actual device pattern to high precision, and a method and apparatus which achieve improved process throughput.
To achieve the above objects, according to this invention, as a technique for measuring film thickness on an actual device pattern, a frequency/phase analysis is performed on a spectral distribution waveform of interference light from light due to a film, and an absolute value of film thickness is computed from a relation between frequency and phase components having a certain waveform and film thickness, or by fitting to a waveform derived from a structural model of the film or a simulation. By calculating a film thickness distribution on an actual device pattern using this measurement technique, high precision film thickness control and process stabilization are achieved. Further, by incorporating a film thickness measurement unit comprising this measurement technique in a polishing apparatus, an improvement of throughput is obtained. The effect of this invention is enhanced by using white light as the light which irradiates the film.
According to this invention, to achieve the above objects, a sample on which an optically transparent thin film is formed on a step pattern is irradiated by light, a reflected light produced by the sample due to this light irradiation is detected, and the film thickness of the optically transparent film formed on the step pattern is calculated based on the spectral distribution waveform of the detected reflected light.
In this way, according to this invention, if the light which irradiates the sample is white light, the film thickness of the optically transparent film can be calculated to a precision of at least xc2x120 nm.